A centrifugal compressor is a simple and reliable machine element when designed as a single stage unit, and is extensively used in gas turbine power plants, turbocharger units and the like, where a compression of 4:1 a 6:1 is satisfactory. In order to obtain a higher degree of compression two or more centrifugal stages will have to be arranged in series, but this means a complication as well as an undesirable increase of weight and requirement for space. The latter consequence is mainly caused by return bends between the individual stages.
It has also been proposed to use one or more axial stages ahead of the centrifugal stage, which may be attractive i.a. with respect to the reduction of costs and weight. In order to obtain any noticeable increase of the pressure with a few axial stages it will however be necessary to design these with a mean diameter which is considerably greater than the outer diameter of the inlet of the compressor. This means a high centrifugal speed in the axial stages and a radial variation of the air distribution and the Mach-number at the centrifugal stage inlet. At the same time the length of the unit will be increased due to the length of the passageway between the axial and the radial parts of the compressor being extended.
A simpler, more compact plant having a high efficiency is desirable with many installations, where a compression ratio of between 6:1 and 12:s is needed. The high compression ratio and the wide operating field of a two stage centrifugal compressor would be advantageous combined with the compactness and the high efficiency of a simple stage axial compressor preceeding a centrifugal compressor. With a wide field of operation this has hitherto been possible with complicated two stage centrifugal compressors only.